There is a class of "crystallizable" polymers useful in the blow mold container art which includes polyesters, such as polyethylene terephthalate (PET), and polyamides, such as nylon-6 and nylon-66. In an amorphous state, such as produced by rapid quenching from the melt, these polymers have relatively low levels of mechanical properties. However, when properly oriented and crystallized, these polymers exhibit a very high tensile modulus and impact resistance enabling the production of a high-strength container at a relatively low weight of material per container.
PET has been in use for over twenty years for carbonated beverage containers, and more recently has been adapted for refillable carbonated beverage containers and hot-fill containers, each of which require a high level of thermal stability. A high thermal stability container can resist distortion and shrinkage when subjected to an elevated temperature during use. A refillable container must withstand a hot caustic wash for cleaning and sterilization, during each reuse cycle. A hot-fill container must withstand an elevated product fill temperature required for sterilization, as is customary with juice and milk products.
Generally, increasing the percent crystallinity of the container increases the resistance to thermal deformation and shrinkage. However, increasing the crystallinity also generally reduces the transparency of the container. In fields such as the carbonated beverage industry where there are strict requirements regarding the transparency of the containers, it is difficult to achieve the required crystallinity levels without also producing opacity or haze in the container.
In the past, the principal mechanism of increasing the crystallinity of PET containers has been by heat setting. Heat setting was performed after the container was expanded, the expanded container being held at an elevated temperature and pressure for a few minutes to increase the crystallinity level and relieve residual stresses. However, the heat setting process adds significantly to the cost of production.
Another method for increasing resistance to distortion caused by hot filling and/or high impacts, is to provide a sidewall having vacuum panels and ribs to reinforce the sidewall. Such a container is described in U.S. Pat. No. 5,178,289 to Krishnakumar et al. entitled "Panel Design For A Hot-Fillable Container," which issued Jan. 12, 1993.
Another method of increasing resistance to deformation caused by hot filling is to add to the container, immediately after filling, liquid nitrogen followed by immediate closing of the container, wherein the liquid nitrogen becomes nitrogen gas and internally pressurizes the container to prevent vacuum collapse. Such a container is described in U.S. Pat. No. 5,104,706 to Krishnakumar et al. entitled "Preform For Hot-Fill Pressure Container," which issued Apr. 14, 1992.
Another method for increasing the thermal stability of a container is to provide a multi-layer construction including inner and outer layers of PET and a central layer of a plastic having a high glass transition temperature (T.sub.g). Such a container is described in U.S. Pat. No. 5,049,345 to Collette et al. entitled "Method Of Forming A Multi-Layer Preform," which issued Sep. 17, 1991. The high T.sub.g polymer may be a copolyester which is preferred due their melt solubility and resulting adhesion to PET. Non-polyesters such as acrylonitrile styrene, acrylonitrile styrene copolymers, polycarbonate, etc. could also be utilized. The higher T.sub.g material may form a central core layer of a three-layer sidewall construction, or inner and outer intermediate layers between inner, core and outer layers in a five-layer construction.
It is an object of this invention to provide a method and apparatus for blow molding a preform into a container which provides an enhanced level of crystallinity in the body of the container in order to provide enhanced thermal stability. This method may be used alone or in combination with prior art methods for enhancing thermal stability, e.g., heat setting, multi-layer construction, vacuum panel design, etc.